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Quiescent Ultra-Diffuse Galaxies in the Field Originating from Backsplash

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Quiescent Ultra-Diffuse Galaxies in the Field Originating from Backsplash This is a pre peer-review version and may differ from the published version in Nature Astronomy. The final authenticated version is available online at https://www.nature.com/articles/s41550-021-01458-1. Quiescent Ultra-diffuse galaxies in the field originating from backsplash orbits Jose´ A. Benavides1;2,* Laura V. Sales3, Mario. G. Abadi1;2, Annalisa Pillepich4, Dylan Nelson5, Federico Marinacci6, Michael Cooper7, Ruediger Pakmor8, Paul Torrey9, Mark Vogelsberger10 and Lars Hernquist11 . 1Instituto de Astronom´ıa Teorica´ y Experimental, CONICET-UNC, Laprida hand, for the few quenched UDGs that have been discovered in the 854, X5000BGR, Cordoba,´ Argentina. 2Observatorio Astronomico´ de field, the mechanism responsible for removing the gas and halting Cordoba,´ Universidad Nacional de Codoba,´ Laprida 854, X5000BGR, star-formation remains unknown. On the theory side, progress requires Cordoba,´ Argentina. 3Department of Physics and Astronomy, University of high resolution cosmological simulations that are able to resolve the California, Riverside, CA, 92521, USA. 4Max-Planck-Institut fur¨ Astronomie, myriad of environments and physics involved in this problem; from the Konigstuhl¨ 17, 69117 Heidelberg, Germany. 5Institut fur¨ theoretische Astro- formation of isolated dwarfs in their haloes, to their interactions with physik, Zentrum fur¨ Astronomie, Universitat¨ Heidelberg, D-69120 Heidelberg, filaments, groups and clusters. Such simulations have only recently Baden-Wurttemburg,¨ Germany. 6Department of Physics and Astronomy become possible, with the TNG50 simulation – used here – among ”Augusto Righi”, University of Bologna, I-40129 Bologna, Italy. 7Department those with the highest resolution available 25, 26. of Physics and Astronomy, University of California, Irvine, 4129 Frederick Reines Hall, Irvine, CA 92697, USA. 8Max-Planck-Institut fur¨ Astrophysik, We use the stellar mass–size relation defined by all simulated Karl-Schwarzschild-Str. 1, D-85748 Garching, Germany. 9Department galaxies (Fig. 1, grey dots) to define our sample of field UDGs as of Astronomy, University of Florida, 211 Bryant Space Science Center, those central galaxies (excluding satellites) with stellar mass in the 10 Gainesville, FL 32611, USA. Department of Physics, Massachusetts dwarf range (log(M?=M ) = [7:5;9], shaded pink region) and a stellar Institute of Technology, Cambridge, MA 02139, USA. 11Institute for Theory size above the 95th percentile at a given mass (magenta stars). Our and Computation, Harvard-Smithsonian Center for Astrophysics, Cambridge, definition overlaps with observational samples of UDGs14, 18, 27, 28. We MA 02138, USA. study the origin of the extended sizes of these galaxies elsewhere (J.A.B. et al., manuscript in preparation) but a brief summary is given Ultra-diffuse galaxies (UDGs) are the lowest-surface bright- in Fig. 6 in the Supplementary Information. ness galaxies known, with typical stellar masses of dwarf galaxies but sizes similar to larger galaxies like the Milky Way1. The Galaxy colours (g-r) of our simulated UDGs in Fig. 2 show a clear reason for their extended sizes is debated, with suggested internal bimodality: most central UDGs are in the ‘blue cloud’, suggesting processes like angular momentum 2, feedback 3, 4 or mergers 5 young stellar populations as expected for dwarfs in the field, while versus external mechanisms 6–9 or a combination of both 10. Ob- 23:7% of our simulated UDGs are along the red sequence. The mass servationally, we know that UDGs are red and quiescent in groups distribution is non-uniform, with red UDGs being more common and clusters 11, 12 while their counterparts in the field are blue and towards the lower masses, where also, at the same mass, field UDGs star-forming 13–16. This dichotomy suggests environmental effects have a higher fraction of red objects than normal dwarfs in the field as main culprit. However, this scenario is challenged by recent (See upper panel of Fig. 6 in the Supplementary Information). The observations of isolated quiescent UDGs in the field 17–19. Here we inset panel shows that their colours correlate with their star formation use LCDM (or L cold dark matter, where L is the cosmological rates, with the blue UDGs occupying the “main sequence” of star- constant) cosmological hydrodynamical simulation to show that forming galaxies and the red UDGs showing negligible star formation isolated quenched UDGs are formed as backsplash galaxies that today. were once satellites of another galactic, group or cluster halo but are today a few Mpc away from them. These interactions, albeit A close inspection of the histories of our red quiescent UDGs arXiv:2109.01677v1 [astro-ph.GA] 3 Sep 2021 brief, remove the gas and tidally strip the outskirts of the dark reveals a factor in common: they have all been satellites of another matter haloes of the now quenched seemingly-isolated UDGs, system in the past but are today central galaxies in the field. The which are born as star-forming field UDGs occupying dwarf-mass top panel of Fig. 3 shows an example of the orbit of one of our red dark matter haloes. Quiescent UDGs may therefore be found in UDGs. This dwarf interacted ∼ 4 billion years ago with a group that non-negligible numbers in filaments and voids, bearing the mark 13 has a virial mass M200(z = 0) t 6:46 × 10 M but is found today of past interactions as stripped outer haloes devoid of dark matter ∼ 1:5 Mpc away, more than twice the virial radius of the group (virial and gas compared to dwarfs with similar stellar content. quantities refer to the radius enclosing 200 times the critical density of the Universe). The colour coding of the orbit, reflective of the Ultra-diffuse galaxies (UDGs) in groups and clusters are charac- dwarf colour at each time, shows that the reddening starts already as it terized by a puzzling wide range of dark matter and globular cluster falls into the group and accelerates after the pericentric passage. The content 20–22, thick disk-like shapes 11, 23, old stellar populations 24 images of the simulated UDG (middle row) clearly show that its gas is and no substantial gas component. Their quiescence is not surprising removed as it approaches the pericenter, explaining its quiescence and given the high density environments they populate. On the other redness today in the field. The stellar size is not largely affected by the interaction, our red UDGs were all already extended before infall (see *E-mail: [email protected] Fig. 5 in the Supplementary Information). 1 2 0.8 c se ar ag m 28 = 0.6 0.4 ] c p k [ 0.2 ) 2 h ec r rs ( a ag g 0.0 5 m o 2 l = 0.2 All simulated galaxies Simulated Central UDGs Isolated UDGs (Rong+20) DGSAT I (Martin-Navarro+19) 0.4 S82 DG1 (Roman+19) UDGs Coma (van Dokkum+15) UDGs Virgo (Lim+20) 7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 9.0 log(M ) [M ] Figure 1 – Definition of the UDG sample. Stellar mass versus size (M? vs rh?) relation for all simulated galaxies in the mass range log(M?=M ) = [7:4;9:1] in the TNG50 simulation (grey dots). Thin blue dashed curves indicate lines of constant surface brightness assuming a mass-to-light ratio equal to 1. The solid black line indicates the median size at fixed M? for the simulated galaxies. Yellow dashed curves show the 5th and 95th percentiles, with the shaded yellow region in between highlighting the sample of normal galaxies. Our sample of field UDGs (magenta stars) is defined as central galaxies with log(M?=M ) = [7:5;9] and stellar size above the 95th percentile (pink shaded region). Several observational data are shown in black edged symbols, where we transform 2D sizes Reff to 3D assuming rh? = 4=3Reff. Light blue diamonds indicate star-forming UDGs in low-density environments 14; the dark blue circle is the relatively isolated DGSAT I [27]; the red pentagon is UDG S82-DG-1, an isolated quiescent UDG 18. For comparison, we also show UDGs in the Virgo cluster 28 (green crosses) and the Coma cluster 1 (pink x-symbols). Our UDG definition agrees well with observational samples, in particular for those in low density environments. remaining cases being “pre-processing”, meaning that the UDG was first quenched in a moderate mass host which subsequently fell into a Objects in such external orbits, which are found far beyond the more massive system responsible for the energetic orbit. virial radius of their hosts, are known as backsplash galaxies 29, and are a natural consequence of the hierarchical assembly in LCDM. Our red UDGs are backsplash objects of systems in a wide range of A section of the simulated box with the location of red and blue 12 virial masses, including galaxy-sized haloes with M200 t 2 × 10 M UDGs is illustrated in Fig. 3 (panel c), highlighting the red UDGs that to galaxy clusters, and are today on average at 2:1r200 from those are backsplash objects of galaxy-size haloes (green circles), located systems, or 1:7 ± 0:7 Mpc, but can reach as far as 3:35 Mpc in some mainly in low-density regions of the Universe. Red field UDGs cluster cases (see Fig. 7 in the Supplementary Information). In the large more than the blue ones, but they are all at substantial distances from majority of cases, (64:3%), the system responsible for the quenching their once-hosts. On average, the interactions occurred ∼ 5.5 Gyr ago and the launching beyond the virial radius is the same, with the and were moderately quick, with red UDGs spending typically 1.5 Gyr 2 1.0 ] a 1 r 0 b All simulated galaxies y M 0.9 Central UDGs Star Forming [ 2 ) Central UDGs Quenched R F 4 Isolated UDGs (Rong+20) S ( g 0.8 S82 DG1 UDG (Roman+19) o l 6 7.5 8.0 8.5 9.0 log(M ) [M ] 0.7 ] g a 0.6 m [ ) r 0.5 g ( 0.4 0.3 0.2 0.1 7.4 7.6 7.8 8.0 8.2 8.4 8.6 8.8 9.0 log(M ) [M ] Figure 2 – Dichotomy in colour and star formation rate of field UDGs.
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